1. Field of the Invention
This invention relates to dielectric ceramics and dielectric resonators for use in such high-frequency ranges as microwave and millimeter-wave frequencies.
2. Description of the Prior Art
Recently, dielectric ceramics have been widely used in dielectric resonators and filters in microwave and millimeter-wave frequencies at wavelengths of several centimeters or less (hereinafter referred to as microwave in general). It is required that a dielectric material for use in such applications have a high unloaded Q (Qu) value and dielectric constant .epsilon..sub.r, and that the temperature coefficient at resonant frequency .tau..sub.f be variable as desired.
Various materials appropriate for use in such applications have been conventionally reported, among which ZrTiO.sub.4 ceramics are included. Also included in such materials are ZrO.sub.2 --SnO.sub.2 --TiO.sub.2 ceramics, the ZrO.sub.2 --SnO.sub.2 --TiO.sub.2 --MgO ceramic suggested in Japanese Laid-Open Patent No. 62-132769 and the ZrO.sub.2 --SnO.sub.2 --TiO.sub.2 --CoO--Nb.sub.2 O.sub.5 ceramic in No. 2-192460 for example.
However, although ZrTiO.sub.4 ceramics have a high dielectric constant of 45, the temperature coefficient at resonant frequency is high in the positive side at 54 ppm/.degree.C., and the temperature coefficient is significantly varied by the heating history during sintering. ZrO.sub.2 --SnO.sub.2 --TiO.sub.2 ceramic systems achieved a low temperature coefficient at a resonant frequency of nearly zero, but the heating history problems were not solved satisfactorily.
In addition, conventional materials have problems such as the dielectric constant and unloaded Q value are low, and that the temperature coefficient at resonant frequency cannot be varied as desired.
Moreover, although the product of resonant frequency (f).times.Qu value is generally regarded as being constant in a given material, when f is lowered (that is, an element is enlarged), actually, the product fQu is reduced (decreased). Therefore, there is a strong demand for a dielectric element for microwave applications such as a dielectric resonator for a base station of mobile radio communication systems used in a relatively low frequency range with a higher unloaded Q value. Furthermore, because dielectric resonators used in the relatively low frequency ranges are very bulky, reduction in size is highly desired.